My b2evolution Version: 2.x

Made myself a new test installation of 2.3.0-rc1 to try it out: worked very well.
Updated my server from php4.3 to 5.1 for other reasons today. B2evo stopped working entirely - blank pages.
Deleted all files and old database (only sample files in it anyway) and made complete new install and new database. Checked permissions for media directories and config file.
Setup went normally.
Logged in with generated admin password but then reached blank page again when I tried to change admin password. Turns out that some of the page is downloading but not enough to show anything - ie. the page isn't empty, the blog is working, kinda. Parts of the admin interface work, parts don't. Nothing useful. Nothing of the front end shows at all.
Reuploaded entire fileset again to ensure no corrupted files. No result.
So here I am again.

Any ideas? FP has said elsewhere that b2evo is fully compatible with php5, but do I need to do anything else different?